Photostructurable glass and ceramic materials that are known as photocerams, can be selectively converted from the glass to metasilicate or ceramic state through exposure to ultraviolet (UV) light followed by baking. The process is a basic expose and bake process. The virgin state is visibly clear glass, the unexposed and baked regions remain visibly clear, and the exposed and baked states become optically dark. These dark regions are in a semi-crystalline metasilicate phase or a full ceramic phase. Formation of the full ceramic phase requires baking to a higher temperature. The exposure process is typically done using a UV lamp with a photomask, or more recently, using a laser direct-write technique. U.S. Pat. No. 6,932,933 teaches use of a laser direct-write technique to create true three-dimensional structures in photocerams. This material absorbs light strongly with wavelengths below 300 nm, so the depth of exposure is a function of the wavelength used during exposure. Light at 350 nm wavelength will expose a centimeter or more deep while a 250 nm wavelength will be limited to a few hundred microns. Energy densities of 20 Joules/cm2 are required to expose this material, and the bake cycle to create the metasilicate phase takes about 8 hours. For example, a basic bake process may have a maximum temperature of 600° C. to convert exposed regions to a dark metasilicate phase while another bake has a maximum temperature of 750° C. to convert exposed regions to the full ceramic state. Processed photostructured glass and ceramic materials have opaque and transmissive optical characteristics. Such photostructurable materials include boron oxide, potassium oxide, silica, aluminum oxide, sodium oxide, zinc oxide, lithium oxide, cerium oxide, antimonium trioxide, and silver oxide, among others.
Standard cathode ray tubes and flat panel displays have a large viewing angle. Many applications require a narrow viewing angle for privacy. Notebook computer displays and other portable displays provide poor viewing under bright ambient light conditions and are difficult to read in sunlight. Plastic add-on privacy screens are available, but they degrade image quality and are not robust enough for outdoor applications such as ATMs. Privacy filters for visual displays are usually composed of a series of optical louvers or a holographic image in a plastic sheet that can be added to the front of a display screen. U.S. Pat. No. 6,765,550 teaches a privacy filter apparatus for a notebook computer display. U.S. Pat. No. 6,731,416 teaches a holographic privacy filter for display devices. More complex designs use two sets of grids or require the user to wear polarization-changing eyeglasses such as taught by U.S. Pat. No. 5,528,319 for a privacy filter for a display device and by U.S. Pat. No. 6,650,306 for a security-enhanced display device. The add-on optical films suffer from image degradation due to the inability to accurately align externally applied baffles with individual pixels while the polarization glasses can be cumbersome.
Lenslet arrays for three-dimensional viewing have been used. U.S. Pat. No. 6,974,216 teaches an autostereoscopic 3-D display. The lenslets must be disadvantageously aligned with individual pixels to prevent Moire interference patterns. Complex parallax barriers have also been used. U.S. Pat. No. 6,970,290 B1 teaches a stereoscopic image display device without glasses, which is not suitable for many hand-held applications where the display screen can be placed in the optimum position for stereoscopic vision. Prior viewing displays suffer from interference pattern generation, degraded viewing images, the required use of special viewing glasses, and nonprivate large fields of view. These and other disadvantages are solved or reduced using the invention.